In general, in an electronic control unit (ECU) of an electrical power steering (hereinafter, referred to as EPS) system of a vehicle, a steering torque resulting from manipulation of a steering wheel by a driver is detected by a torque sensor so as to accomplish the steering operation.
In supplying power to the torque sensor for detecting the steering torque in the conventional EPS system, a battery voltage is supplied to the ECU, the input voltage passes through a regulator in the ECU, and the voltage output from the regulator is used as a source voltage of the torque sensor.
However, when the battery voltage drops lower than a predetermined voltage due to a temporary overload of the vehicle, the source voltage of the torque sensor output from the regulator in the ECU is lower than an allowable range of a rated voltage and thus the torque sensor does not normally work, so that there occurs a problem that the steering operation is not accurately accomplished due to decrease in output of the torque sensor.